Abstract: Polymer-based dielectric nanocomposites have received a lot of attention recently because of their strong plasticity, high power density, and excellent stability; however, they cannot satisfy the specifications of high-precision energy storage devices in the current electrification era due to their lower energy density and constrained breakdown resistance. In order to obtain a polymer-based composite with both high breakdown strength and high energy storage density, in this paper, a novel sandwich structure composite was created by blending the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene-chlorofluoro ethylene) (PVTC) with the linear polymer polymethyl methacrylate (PMMA) as the external insulating layer, and PVTC was filled with the core-shell nanofiber BT@AO@PDA NFs with high dielectric constant as the high dielectric polarization layer. The positive sandwich structure (insulation layer-polarization layer-insulation layer) realizes the cooperative improvement of breakdown strength and dielectric constant. Among them, the positive sandwich structure composed of PMMA/PVTC (25% PMMA) composite and 3% BT@AO@PDA/PVTC composite achieves a high energy storage density of 17.25 J/cm³ under the electric field of 562.7 MV/m, and also has excellent charge-discharge cycle stability and ultra-fast discharge rate of 1.42 μs. This work offers a quick and efficient technique for manufacturing flexible dielectric film capacitors with high energy storage.